The Annelida are segmented worms in which
the body wall, coelom (body cavity), epidermis,
circular muscle, longitudinal muscle, and
peritoneum are arranged into a longitudinal series
of rings or segments. True segmented animals
exhibit metamerism, a repetition of a structure or
organ from segment to segment. Each segment
has the same fundamental structures as all the
others.With the exception of the digestive system,
the major organ systems of the annelids are metameric
in structure. Young annelids generally have
few segments, but as they grow, new segments are
formed by the division of the terminal segment.
Annelids represent the most highly organized animals
capable of complete regeneration.
General Characteristics of the Annelids
The mouth lies between the first and second segments and forms one segment called the prostomium. In leeches, the mouth contains suckers for attaching to the body of a host. The brain originates in the prostomium and develops a pair of circumpharyngeal nerve rings that reach around the pharynx to form the ventral nerve cord, which appears as a chain of ganglia, one pair in each segment. In the Polychaetes, a pair of swimming or crawling parapodia are located on most of the segments. Both the Polychaetes and Oligochaetes contain external setae to assist in locomotion. The annelid body is covered with a thin cuticle. Each segment has a ring of circular and longitudinal muscles that contract to either elongate or shorten the segment. Aspacious coelom, divided by septa, lies between the body wall and an internal digestive tract. The coelom is filled with fluid and serves as a hydrostatic skeleton in all annelids except the leeches. The coelom also contains the circulatory and excretory systems. A system of large vessels (hearts) pump blood through a ventral vessel into capillary beds that invade all of the tissues. The blood is returned to the hearts via the dorsal vessel. Each segment, except the first and last, contains a pair of nephridia, which collect wastes and deliver them to the outside.
Reproduction in the Annelids
In many species of polychaete worms, fertilization is external and takes place in the open sea water. The palolo worm provides a good example of polychaete reproduction. Through most of the year, the worms exist as sexually immature animals called atokes, but during the breeding season, the posterior segments develop gonads, and the coelom becomes filled with gametes. On the night of breeding, individuals back out of their holes, and the posterior portion, called an epitoke, breaks free. The epitokes swim to the surface for a few minutes and burst, shedding eggs or sperm and leaving a rapidly disintegrating body. Reproduction in the palolo worms is tied to an annual cycle designating the month, a lunar rhythm designating the day, and a diurnal cycle designating the hour of reproduction. Over 90 percent of the population breeds within a single two-hour period of the entire year. Oligochaete worms are hermaphroditic, with a pair of testes in the tenth and eleventh segments and a pair of ovaries in the thirteenth segment. During copulation, two worms exchange sperm. Once the eggs are fertilized, the clitellum, a swollen glandular region of the epidermis, secretes a membrane that slips forward along the body so that the eggs are laid directly into it as it passes. Finally, the cocoon slips off the head, and the eggs develop into tiny worms, which later emerge from the cocoon. The reproductive system of leeches is very similar to that of the Oligochaetes.
Classes: Polychaeta (with parapodia and numerous setae), Archiannelida (small marine annelides with simple body), Oligochaeta (parapodia absent and few setae), Hirudinea (leeches, parapodia and setae absent)
Orders: Polychaeta—Errantia (palolo worms), Sedentaria (lugworms); Oligochaeta—Lumbricus (earthworms); Hirudinea—Rhynchobdellida (no jaws, pharynx eversible, colorless blood), Gnathobdellida (three jaws, red blood)
Geographical location: Found all over the world
Habitat: Polychaeta—mostly marine, found near the shore or on the bottom of shallow areas, with a few species in living in brackish water or freshwater; Archiannelida—marine; Oligochaeta—freshwater and terrestrial forms found burrowing in soil or leaf mold; Hirudinea—freshwater and terrestrial environments often found attached to the body of a host
Gestational period: Varies among species, but most species lay eggs within a few days after fertilization; eggs usually hatch within a few days to a few weeks after being deposited
Life span: Varies among species; can be as short as a year in some polychaetes and up to several years in some earthworms
Special anatomy: Elongated, metameric (segmented) bilateral invertebrates with appendages (parapodia) and chitinous setae in many species but lacking in Hirudinea; possess a true coelom (body cavity lined with epithelial tissue) divided by septa with a closed circulatory system, complete digestive system and an excretory system (nephridia) in each segment; leeches have specially adapted mouth parts for attaching to the body of the host
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